Boyd’s Bible Dictionary:

(behold a son!). Eldest son of Jacob and Leah (Gen. 29:32). Lost his birthright through crime (Gen. 35:22; 49:3-4). Tribe numerous and pastoral, and settled east of Jordan (Num. 1:20-21; Josh. 13:15-23). Idolatrous, averse to war, carried captive by Assyria (Judg. 5:15-16; 1 Chron. 5:26).

Concise Bible Dictionary:

The firstborn of Jacob and of Leah, and head of one of the twelve tribes. The territory they possessed also bears his name. He saved the life of Joseph when his brothers thought to kill him, and when they went to buy corn in Egypt, he offered to be responsible for Benjamin’s safety. Jacob, when blessing his sons, said, “Reuben, thou art my firstborn, my might, and the beginning of my strength, the excellency of dignity, and the excellency of power: unstable as water, thou shalt not excel; because thou wentest up to thy father’s bed; then defiledst thou it” (Gen. 49:3-4). This speaks of failure in the firstborn, and implies loss of his birthright. (Joseph, type of Christ separated from His brethren, had the birthright.) Moses, when he blessed the tribes (showing more their relationship with God according to His government) said, “Let Reuben live, and not die: and let not his men be few” (Deut. 33:6). Reuben entered Egypt with his four sons, Hanoch, Phallu, Hezron, and Carmi (Gen. 46:9).
At the Exodus the tribe numbered 46,500 men fit to go to war; and at the close of the wanderings they had decreased to 43,730. At their request, Reuben had their possession on the east of the Jordan, because it was “a place for cattle.” It extended northward from the river Arnon about 25 miles, where it joined the possession of Gad.
The Reubenites do not appear to have taken any prominent part in the struggles under the Judges; they had “great thoughts of heart,” but remained with their flocks (Judg. 5:15-16). They made inroads upon the Bedouin tribes: being on the border of the wilderness doubtless this was unavoidable if they were to live in peace and safety (1 Chron. 5:9-10, 18; &c). The Reubenites, with the others on the east of the Jordan, went after the gods of the heathen, and Jehovah cut them short by Hazael, king of Syria (2 Kings 10:32-33). Afterward by Pul and Tiglath-pileser they were carried away captive unto Halah, Habor, Hara, and to the river Gozan (1 Chron. 5:26).
The east of the Jordan was a place of danger. Remaining there was a type of a Christian stopping short of the place of nearness God has given him—not realizing his death and resurrection with Christ, and his true place in the heavenlies.

Jackson’s Dictionary of Scripture Proper Names:

see ye, a son